Hi Patvp,
Your grandfathers Army record would be held at The National Archive. However, many of the records of soldiers from WWI were destroyed in a fire during a bombing raid during WW2 and it is estimated that only 60% survive and some of these are not in a great state. They are known as the burnt records.
These documents are being added to Ancestry, but you have to pay to view these. At the moment the coverage is a bit patchy as they are still adding to this database.
There are other sources that you may gain information from:
Comonwealth Grave Commission Web Site - has details of your grandfather.
WWI Medal cards - available on The National Archive website has these at £3.50 to download, they are also available on Ancestry (not complete).
Soldiers Died in Great War - is available on one of the pay-to-view websites (I am sure that a Roots Chater in the WWI area would be able to tell you.)
Another useful source could be local newspapers which can be available in local family history libraries. These often contain pictures of soldiers and in some cases small articles. It can however be a bit hit or miss and like looking for a needle in a hay-stack.
Hope this helps
Ore Dragon